An implantable cardiac stimulation device is a type of implantable medical device (IMD) that delivers therapy to the heart of a patient in which the device is implanted. For example, a pacemaker recognizes various cardiac arrhythmias and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD additionally or alternatively recognizes ventricular tachycardia (VT) and ventricular fibrillation (VF) and delivers electrical shocks or other therapies to terminate these tachyarrhythmias. At least some pacemakers and ICDs are also equipped to deliver CRT. Briefly, CRT seeks to normalize the dyssynchronous cardiac electrical activation and resultant dyssynchronous contractions associated with congestive heart failure (CHF) by delivering synchronized pacing stimulus to both sides of the heart using left ventricular (LV) and right ventricular (RV) leads. The stimulus is synchronized to improve overall cardiac function. This may have the additional beneficial effect of reducing the susceptibility to life-threatening tachyarrhythmias.
For the purposes of detecting and responding to various arrhythmias, the implantable device tracks the heart rate of the patient by examining electrical signals associated with the contraction and expansion of the chambers of the heart. The contraction of atrial muscle tissue is triggered by the electrical depolarization of the atria, which is manifest as a P-wave in a surface electrocardiogram (ECG) and as a rapid deflection (intrinsic deflection) in an intracardiac electrogram (IEGM). The contraction of ventricular muscle tissue is triggered by the depolarization of the ventricles, which is manifest on the surface ECG by an R-wave (also referred to as the “QRS complex”) and as a large rapid deflection (intrinsic deflection) within the IEGM. Repolarization of the ventricles is manifest as a T-wave in the surface ECG and a corresponding deflection in the IEGM. A similar depolarization of the atrial tissue usually does not result in a detectable signal within either the surface ECG or the IEGM because it coincides with, and is obscured by, the R-wave. Note that the terms P-wave, R-wave and T-wave initially referred only to features of a surface ECG. Herein, however, for the sake of brevity and generality, the terms are used to refer to the corresponding signals or deflections sensed internally. Also, where an electrical signal is generated in one chamber but sensed in another, it is referred to herein as a “far-field” signal. The misidentification of far-field signals as near-field events is referred to as far-field oversensing (FFOS).
The sequence of electrical events that represent P-waves followed by R-waves (or QRS complexes) followed by T-waves can be detected within IEGM signals sensed using pacing leads implanted on or within the heart. To help prevent FFOS and to more accurately detect the heart rate, the stimulation device employs one or more refractory periods and blanking periods. Within a refractory period, the device does not process electrical signals during a predetermined interval of time—either for all device functions (an absolute refractory period) or for selected device functions (a relative refractory period). As an example of a refractory period, upon delivery of a V-pulse to the ventricles, a post-ventricular atrial refractory period (PVARP) is applied to an atrial sensing channel. A first portion of the PVARP comprises a post-ventricular atrial blanking (PVAB) interval (which can also be referred to as an absolute refractory period). The PVAB is primarily provided to prevent the device from erroneously responding to far-field R-waves on the atrial channel. The PVARP concludes with a relative refractory period during which the pacemaker ignores all signals detected on the atrial channel as far as the triggering or inhibiting of pacing functions is concerned but not for other functions such as detecting rapid atrial rates or recording diagnostic information. As another example of a refractory period, upon delivery of the V-pulse to the ventricles, a ventricular refractory period (VREF) is applied to LV and RV sensing channels for preventing evoked responses (ERs) triggered by the V-pulse from being misidentified as R-waves and also for preventing the T-waves of intrinsic (i.e. non-paced) beats from being misidentified. Despite the use of PVARP and VREF intervals, FFOS can nevertheless still arise, with consequences ranging from benign to dangerous.
In particular, FFOS may arise due to incorrectly set refractory and blanking periods or due to incorrectly programmed sensitivity values. FFOS may be more likely in some anatomic configurations of the leads or in some cases of aberrant conduction. Among the adverse consequences of FFOS are inappropriate tracking of higher rates leading to pacemaker mediated tachycardia (PMT) and inappropriate mode switch leading to loss of atrial contribution to ventricular function. Moreover, FFOS in the ventricles can lead to inappropriate tachycardia therapy (either anti-tachycardia pacing (ATP) or shock therapy) that can be a prognosticator of decreased survival.
Improved techniques for correctly detecting and rejecting FFOS would be highly advantageous, and it is to this end that aspects of the invention are generally directed.